EP3127137B1 - Method and device for generating a microwave plasma in the field of electronic cyclotronic resonance - Google Patents

Description

The invention relates to the technical sector of the production of plasma by electron cyclotron resonance (ECR) from a gaseous medium.

More particularly, the invention relates to the surface treatment under vacuum by plasma of any type of filiform elements such as threads, tubes, fibers and more generally of any product whose length is very large compared to the diameter. The filiform element being continuously linearly driven.

By plasma vacuum surface treatment is meant cleaning, pickling, activation, grafting of functions or coating of the surface, for example by PECVD (plasma-assisted physical vapor deposition) of the filiform element.

Numerous technical solutions are known for producing microwave applicators for treating different types of parts. We can cite for example, by way of indication and without limitation, the teaching of the patent EP 1075168 which relates to a method and a device for producing elementary plasmas with a view to creating a uniform plasma for a surface of use. We can also cite the teaching of the patent FR 2 922 358 which relates to a method of surface treatment of at least one part by means of elementary sources of plasma by electron cyclotron resonance. The various solutions covered by these patents are particularly suitable for the treatment of large surfaces or batches of parts placed one beside the other and generally having multiple faces to be treated.

According to the prior state of the art, using a microwave applicator with a magnetic tip, it appears that the plasma is generated at the end of each magnet creating a dense zone of plasma. It is also known that in order to generate a microwave plasma at low pressure, the effect of electron cyclotron resonance is used. The probability of high speed shocks is greatly increased which creates a dense plasma in the RCE zone. Thus, for a frequency of 2.45 GHz, the RCE zone is located at the level of the magnetic field lines at 875 Gauss (G). This 875 Gauss (G) area is around the magnet.

This plasma applicator technology is not suitable for the continuous treatment of a wire (or other filiform element) requiring several applicators placed radially and repeated several times along the axis of travel of the wire to be treated to obtain a speed of scrolling.

In fact, the volume of plasma being localized punctually at the end of the applicators, it is necessary to use several applicators all around the wire (or other filiform element) to guarantee uniform axisymmetric deposition. Such a configuration requires a large deposition chamber, which consumes gas and energy. The multiplication of applicators and the lack of compactness make this system expensive to build.

It therefore appears that the juxtaposition of conventional RCE sources does not make it possible to obtain a plasma configuration favorable to deposition on a filiform element.

For the treatment of son under vacuum, according to the state of the art, treatments of the PVD type (physical vapor deposition), physical vapor deposition, as is apparent for example from the teaching of documents have been proposed. WO 2005/095078 , WO 2006/002673 , FR 2667616 and EP 1231292 , EP 1277874 .

We also know the patent US 6,638,569 according to which a conventional vacuum chamber is used and the wire is subjected to multiple round trips in said chamber so as to expose the maximum surface area of the wire to the plasma. This solution is not very effective because the surface of the wire is negligible compared to the size of the enclosure and is relatively complex by implementing return systems operating under vacuum.

From this state of the art, the desired aim is to be able to carry out on any type of filiform element a plasma vacuum surface treatment as defined above. According to the teaching of the patent US 5,595,793 , a PECVD coating, for example a carbonaceous coating, is deposited on a fiber using a surface microwave plasma wave to generate the plasma. However, this solution is very limited in application given that it can only operate on dielectrics and only for producing electrical insulating deposits. In other words, it is not possible to coat conductive fibers. Furthermore, the frequency of the generator must be adapted to the dielectric constant of each material constituting the fiber. The process is therefore not easily transferable when passing from one material to another. Finally, the process is difficult to master because as the deposition takes place, the dielectric constant of the material changes. This modification has a retroactive effect on the coupling of the surface wave with the plasma.

It therefore emerges from this analysis of the state of the art that the generation of plasma using the applicators is not suitable for the continuous treatment of filiform elements, the volume of the enclosure being oversized with respect to the size of the element, the precursor gas and the energy required being large while the plasma is not generated near the wire to be coated. It also emerges that alternative microwave plasma techniques based on surface waves are limited in their applications and difficult to implement.

We also know the document US 2011/079582 , in which magnets are arranged in an enclosure filled with gas at a pressure below atmospheric pressure. A thread-like element is fixedly supported in the tube, which is movable relative to the magnets. A microwave energy source is mounted outside the enclosure. Plasma rings are created around the magnets, making it difficult to cool them. This device does not make it possible to generate a linear plasma confined around the filiform element in the treatment chamber formed by the tube.

We finally know the document US 2002/172780 , in which non-annular magnets are mounted on either side of a treatment chamber. The displacement of the filiform element is carried out in a direction parallel to the lines of magnetic fields, which corresponds to the state of the art shown in figure 1 of the present application. This device does not make it possible to guarantee the homogeneity of the treatment on the filiform element.

The aim of the invention is to remedy these drawbacks in a simple, safe, efficient and rational manner.

The problem which the invention proposes to solve is to allow the generation of a linear plasma confined around any type of filiform element as defined, in order to minimize the volume of the chamber and, consequently, the investment of consumption of precursor gas and of the necessary energy with the aim of generating axisymmetric plasma in order to guarantee the homogeneity of the treatment on the part, in particular by PECVD.

To solve such a problem, a method according to claim 1 has been designed and developed.

The invention also relates to a device according to claim 4.

It results from these characteristics that the size of the device (reactor) is reduced, consequently reducing the investments allowing a reduction in gas consumption. It is also observed that the densest plasma is found on the wire and no longer close to the latter as is apparent from the solutions relating to the prior state of the art, thus allowing an increase in the deposition rate. These characteristics also make it possible to obtain a homogeneous deposit on the wire taking into account the axisymmetry of the magnetic field lines. It should also be noted, as regards a plasma treatment in order to carry out a chemical deposition, that better use of the monomer is obtained and less rapid fouling of the walls of the reactor.

• Les aimants annulaires peuvent être des aimants permanents, des bobines électromagnétiques, ou tout autre moyen permettant de créer un champ magnétique.
• L'applicateur micro-onde est disposé perpendiculairement à l'axe du tube.
• Le tube constitue un Té dont la branche médiane reçoit l'applicateur micro-onde tandis que les deux autres branches reçoivent les aimants de part et d'autre de ladite branche médiane.

According to other characteristics:

• The ring magnets can be permanent magnets, electromagnetic coils, or any other means making it possible to create a magnetic field.
• The microwave applicator is placed perpendicular to the axis of the tube.
• The tube constitutes a T whose middle branch receives the microwave applicator while the two other branches receive the magnets on either side of said middle branch.

The dimensioning of the ring magnets must be such that the magnetic field in the center of the system between two magnets must be equal to the magnetic field at electronic cyclotron resonance.

For example if the annular magnets are coils of radius R comprising n turns traversed by a current of amperage I, the distance D which separates these two coils must be such that: $$\frac{m.\omega }{e}=\frac{{\mu }_0.\mathrm{not}.I}{R}{\left(\frac{R^2}{R^2+{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}{2}\right)}^2}\right)}^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

Where m is the mass of electrons, e their charge and ω the pulsation of the microwave wave.

We recognize in the term to the right of this equation, the Biot and Savart equation.

In one embodiment, the device comprises several modules mounted in series in linear alignment and interconnected by a sealing ring. Each ring acts either as a pumping zone by being connected to a gas pumping connector or as a gas injection zone being connected to gas supply devices.

Note that the filiform element can be electrically polarized in order to allow bombardment by ions from the plasma. When the filiform element is polarized, it is possible to carry out an ion implantation of a gas on said element.

• La fig. 1 montre un schéma de principe d'un réacteur selon l'état antérieur de la technique pour générer un dépôt sur un fil à revêtir ;
• La fig. 2 est une vue correspondant à la figure 1 montrant le principe du dispositif selon l'invention ;
• La fig. 3 est une vue en perspective d'un module de base du dispositif selon l'invention ;
• La fig. 4 est une vue en perspective montrant le montage de plusieurs modules du dispositif pour augmenter la vitesse de traitement,
• La fig. 5 est une courbe des analyses FITR montrant de façon très classique que le dépôt s'approche d'autant plus du SiO₂ que la ration O₂/HMDSO est élevé.

The invention is explained below in more detail with the aid of the figures of the appended drawings in which:

• The fig. 1 shows a block diagram of a reactor according to the prior state of the art for generating a deposit on a wire to be coated;
• The fig. 2 is a view corresponding to the figure 1 showing the principle of the device according to the invention;
• The fig. 3 is a perspective view of a basic module of the device according to the invention;
• The fig. 4 is a perspective view showing the mounting of several modules of the device to increase the processing speed,
• The fig. 5 is a curve of the FITR analyzes showing in a very classic way that the deposit approaches the SiO ₂ all the more as the O ₂ / HMDSO ration is high.

As indicated, the invention finds a particularly advantageous application for generating a plasma with a view to the surface treatment of any type of filiform element, including a conductor, of the type of wires, fibers, tubes, sheaths, etc. and more generally any element. (F) having a significant length compared to its diameter. The object sought according to the invention is to continuously treat the element (F) in the “parade”, in other words, by linear driving of the wire.

According to the invention, the device or reactor comprises at least one module composed of two magnetic dipoles (1) and (2) arranged opposite and preferably mounted around a tube (3), constituting a treatment chamber. Each magnetic dipole (1) and (2) is for example constituted by an annular magnet arranged concentrically to the tube (3). This assembly facilitates in particular the cooling of the magnets. In fact, unlike the RCE applicators described in the state of the art, the magnets are not under vacuum. The element (F) is engaged coaxially with the tube (3) and continuously linearly driven by any known and appropriate means. A microwave applicator (4), of any known and appropriate type, is mounted between the two magnets (1) and (2). The microwave applicator (4) is arranged perpendicular to the axis of the tube (3). Preferably the opposite polarities are opposite so that the field lines are parallel to the element F. We refer to the figure 2 which shows that the plasma at the RCE zone is on the wire. There is also an axisymmetry of the magnetic field lines (C) making it possible to achieve a homogeneous deposit on the element (F).

In one embodiment, the tube (3) constitutes a T whose middle branch (3a) receives the microwave applicator (4) in particular, its coaxial guide. (4a). The other two branches (3b) and (3c) of the tee receive the magnets (1) and (2) on either side of the middle branch (3a).

From this basic design of the device, it is possible to mount in series and in linear alignment several modules as shown in figure (4 ). In this configuration, the connection between the modules is ensured by a sealing ring (5) which also acts as a pumping zone by being connected to a gas pumping connector (6). In this configuration, the plasma and possibly reactive gases are preferentially injected opposite the microwave applicators (injection not shown in the figure). An alternative configuration to that shown consists in having the sealing rings alternately act as a pumping zone and a gas injection zone.

The pumping is distributed between the center of the reactor and the right and left ends of the latter. The filiform element (F) is introduced linearly into the treatment chamber resulting from the tube constituted by a linear alignment and the series assembly of the various branches (3b), (3c) of the tubes and the rings (5). To increase the running speed of the threadlike element (F), it suffices to multiply the number of modules.

Note that it is possible to inject, into each module, a suitable precursor and to laminate the pumping circuits to adjust the working pressures of each module.

Tests were carried out with Cobalt samarium magnets (Sm ₂ Co ₁₇ ) without excluding any other material to generate a magnetic field of 875 G such as neodymium iron boron.

These tests were carried out according to two configurations.

First configuration :

• diamètre interne 20 mm,
• diamètre externe 28 mm,
• épaisseur 20 mm, polarisation suivant l'épaisseur,
• distance entre les aimants 31, 5 mm
• polarités opposées entre les aimants.

The magnets have the following dimensions:

• internal diameter 20 mm,
• external diameter 28 mm,
• thickness 20 mm, polarization according to the thickness,
• distance between magnets 31.5 mm
• opposite polarities between the magnets.

Second configuration :

• diamètre interne 33,8 mm,
• diamètre externe 50 mm,
• épaisseur 25 mm, polarisation suivant l'épaisseur,
• distance entre les aimants 46 mm
• caractéristique du tube servant de chambre de traitement : DN25 soit 33,7 mm de diamètre extérieur
• polarités opposées entre les aimants.

The magnets have the following dimensions:

• internal diameter 33.8 mm,
• external diameter 50 mm,
• thickness 25 mm, polarization according to the thickness,
• distance between magnets 46 mm
• characteristic of the tube serving as treatment chamber: DN25, i.e. 33.7 mm outside diameter
• opposite polarities between the magnets.

• Les micro-ondes sont injectées au centre de l'espace entre les deux aimants. La profondeur de pénétration de l'injecteur micro-onde doit être optimisée pour faciliter l'amorçage et le fonctionnement du plasma.
• Les aimants sont à la pression atmosphérique. Les aimants sont refroidis par contact avec une enveloppe externe dans laquelle circule un fluide par exemple de l'eau. Les zones de pompage et les zones d'injection de gaz ont été alternées.
• Les aimants sont maintenus dans le système par trois vis de pression pour éviter qu'ils ne s'attirent.

In these two configurations:

• The microwaves are injected into the center of the space between the two magnets. The penetration depth of the microwave injector must be optimized to facilitate the initiation and operation of the plasma.
• The magnets are at atmospheric pressure. The magnets are cooled by contact with an external envelope in which circulates a fluid, for example water. The pumping zones and the gas injection zones have been alternated.
• The magnets are held in the system by three set screws to prevent them from attracting each other.

• la génération d'un plasma linéaire confiné autour de l'élément à traiter afin de minimiser le volume de la chambre et par conséquent diminuer les investissements et la consommation du gaz précurseur et de l'énergie,
• la génération d'un plasma axisymétrique afin de garantir l'homogénéité du dépôt sur l'élément à traiter,
• la possibilité de traiter tout type d'éléments filiformes y compris conducteur du type fils, tubes, fibres et plus généralement tout produit dont la longueur est importante par rapport à son diamètre.

The advantages emerge clearly from the description, in particular, we underline and recall:

• the generation of a linear plasma confined around the element to be treated in order to minimize the volume of the chamber and consequently reduce the investments and the consumption of the precursor gas and of the energy,
• the generation of an axisymmetric plasma in order to guarantee the homogeneity of the deposit on the element to be treated,
• the possibility of treating any type of filiform element including conductor of the type son, tubes, fibers and more generally any product whose length is important compared to its diameter.

The invention is defined by the claims.

By way of example, there is described below tests for depositing SiO _x by PECVD ECR in a reactor according to the second configuration.

• Débit de TMS (Tétraméthyl Silane) : 5 sccm
• Débit de d'O₂ (dioxygène) : 18 sccm
• Pression : 1,3.10^-2 mbar
• Puissance d'injection des micro-ondes : 100 W

First PECVD process

• TMS (Tetramethyl Silane) flow rate: 5 sccm
• O ₂ (oxygen) flow rate: 18 sccm
• Pressure: 1.3.10 ^-2 mbar
• Microwave injection power: 100 W

With this O ₂ / TMS ratio of 3.6, the deposition rate observed between the two magnets in the middle of the chamber is 250 nm / min.

The deposition rate is measured on a silicon wafer placed in the center of the reactor.

• Pression : 1.10^-2 mbar
• Puissance d'injection des micro-ondes : 50 W

Second PECVD process

• Pressure: 1.10 ^-2 mbar
• Microwave injection power: 50 W

Using an O ₂ / HMDSO mixture O ₂ / HMDSO ratio Deposition rate nm / min 9 530 3 875 1.7 1100

Claims (12)

1. Process to generate a plasma excited by microwave energy in the field of electron cyclotron resonance, ECR, to execute a surface treatment or coating around a filiform element (F), according to which:

   - at least two annular magnets (1, 2), each constituting a magnetic dipole, are arranged at atmospheric pressure, facing each other, and around a tube (3) constituting a treatment chamber,

   - the filiform element (F) is linearly and continuously moved through the at least two annular magnets (1, 2) and through the tube (3) forming the treatment chamber,

   - microwave energy is introduced between the at least two annular magnets (1, 2) via a microwave applicator (4) mounted between the at least two annular magnets (1,2),

   - a confined linear plasma is generated around the filiform element (F) in the treatment chamber.
2. Process according to claim 1, wherein the surface treatment is a cleaning, a scouring, a functionalisation, or a grafting.
3. Process according to claim 1, wherein the coating is obtained by PECVD (plasma-enhanced chemical vapour deposition).
4. Device to generate a plasma around a filiform element (F) driven linearly and continuously and comprising means of production of a microwave energy in the field of electron cyclotron resonance, ECR, and a microwave applicator (4), the device further comprising a tube (3) constituting a treatment chamber, at least one module composed of two annular magnets (1, 2), each constituting a magnetic dipole, arranged at atmospheric pressure, facing each other and mounted around the tube (3) ; device in which the tube (3) and the two annular magnets (1, 2) are configured in such a way that the filiform element (F) to be treated is linearly movable through the two annular magnets (1, 2) and through the tube (3) constituting the treatment chamber, and the microwave applicator (4) is mounted between the two annular magnets (1, 2) to introduce microwave energy between the two annular magnets (1, 2), thereby generating, when the device is in use, a linear plasma confined around the filiform element (F) in the treatment chamber.
5. Device according to claim 4, wherein the annular magnets are permanent magnets.
6. Device according to claim 4, wherein the annular magnets are electromagnet coils.
7. Device according to claim 4, wherein the microwave applicator (4) is arranged perpendicularly to the axis of the tube (3).
8. Device according to claim 4, wherein the tube (3) constitutes a Tee whose median branch (3a) receives the microwave applicator whereas the other two branches (3b, 3c) receive the annular magnets (1, 2) on each side of said median branch (3a).
9. Device according to any one of claims 4 to 8, comprising several modules mounted in series and in linear alignment and connected together by a sealing ring (5).
10. Device according to claim 9, wherein each sealing ring (5) is connected to a gas pumping connector such that each sealing ring acts as a pumping zone, when the device is in use.
11. Device according to claim 9, wherein the sealing rings (5) are arranged so as to act alternately as a gas pumping zone and injection zone when the device is in use.
12. Device according to any one of claims 4 to 11, wherein the filiform element (F) is electrically polarised to allow a bombardment from the plasma ions.

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